Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A typical ion implantation process uses an energetic ion beam to introduce impurities into the semiconductor wafers. As is well-known, introducing the impurities at a uniform depth and density into the wafers is important to ensure that the semiconductor devices being formed operate within specification.
One factor in the ion implantation process that can affect the uniformity of the impurity dose in the wafer is vacuum fluctuations during the implantation process. The vacuum fluctuations can be caused by photoresist or other materials coated on a semiconductor wafer that outgas, volatilize or sputter when the ion beam impacts the semiconductor wafer. The outgassing, volatilization or sputtering releases gas particles, which cause a pressure rise in the normally high vacuum condition along the beam line and can result in collisions between ions in the beam and released particles. These collisions can cause ions in the beam to experience a charge change. For example, singly-charged positive ions in an ion beam may collide with residual gas atoms produced by photoresist outgassing during implantation, and experience a charge exchange without a significant change in kinetic energy. The singly-charged positive ions may be neutralized by the collisions and impact the semiconductor wafer in the neutral charge state. In contrast, when outgassing, volatilization or sputtering does not occur from the semiconductor wafer surface, the vacuum level can remain relatively high and constant along the beam line, thus resulting in fewer ion charge exchanging collisions.
The charge exchanging collisions that result when the vacuum level along the beam line drops can cause problems because the detectors used to determine and control the ion beam current (and also the total dose of the wafer) during implantation typically only detect charged particles, but not neutral particles. The neutral particles that are implanted in the wafer are the desired implantation species and have the desired energies for implantation and thus, should be counted in the total implant dose. Since the typical ion beam current detector, such as a Faraday cup, is not capable of detecting the neutral particles, neutral particles that should be counted as contributing to the wafer dose are not detected. As a result, a beam current that is less than the actual beam current is detected, thereby prompting an increase in the beam current and overdosing of the wafer.
Previous methods for controlling implantation uniformity during vacuum fluctuations include detecting both the ion beam current and the vacuum level in the implantation chamber and controlling the ion beam accordingly, as disclosed in U.S. Pat. No. 4,587,433 to Farley and U.S. Pat. No. 5,760,409 to Chen. Such systems have drawbacks, including improper control caused by a delay between an actual change in vacuum along the beam line and the time when the vacuum change, i.e., a pressure change, is detected. This delay between actual vacuum change and detection can cause a delay in ion beam control and result in improper wafer dosing. This type of method also has the disadvantage of requiring the empirical correlation of a plurality of detected gas pressure and beam current values with a corresponding correction value for a plurality of sets of ion implantation parameters, such as gas composition, beam energy, implant species, amount of dose, photoresist type, etc.
Accordingly, a method for controlling implantation during vacuum fluctuation that is independent of such implantation parameters and that can rapidly respond to vacuum fluctuation is needed.